1. Field of the Invention
The present invention relates generally to an amplifier which is connectable to a selected one of a plurality of input circuits through a selected one of a plurality of negative feedback circuits, and more particularly is directed to an amplifier with switchable inputs and feedback circuits.
2. Description of the Prior Art
In the art, an amplifier having negative feedback is employed for high gain. In such a prior art negative feedback amplifier, an output signal from a phonograph or an output signal from a stereo-tuner is selected as a signal source by a signal switch. The amplifier is used to obtain necessary amplification for the respective input signals and to control the frequency characteristic of a phonograph output signal in accordance with pre-emphasis characteristics and de-emphasis characteristics (RIAA-characteristics) during recording. A feedback switch is used to connect a phono feedback circuit with a pre-emphasis frequency response to the amplifier to obtain a flat signal at the output of the amplifier when a phono input is used. A feedback circuit for the stereo-tuner output signal is simultaneously opened.
In the prior art there is a fear that oscillations will occur resulting from positive feedback to the amplifier input by a leakage resistance existing between the contacts of the feedback switch and the signal switch, the contacts of each switch being near each other in a function selector. For this reason, prior art circuits require a function selector containing the signal and feedback switches which has a good deal of distance between its contacts. Accordingly, a function selector of relatively large size is used and the freedom of design is limited. Also, the wiring layout is critical.